Shuangying Wei

Preparation and performance of spherical FeF2.5·0.5H2O nanoparticles wrapped by MWCNTs as cathode material of lithium ion batteries

Spherical FeF2.5·0.5H2O–MWCNTs nanocomposites are synthesized via an ionic liquid (IL) based on precipitation route as a high performance cathode material for lithium ion batteries (LIBs), in which the BMMimBF4 is used as environmentally friendly fluorine source, appropriate solvent and binder. The addition of MWCNTs can not only increase the conductivity of the active material, but also play a role in designing a especial morphology where nano-sized FeF2.5·0.5H2O particles grew up. The structure, morphology and electrochemical performance of the as-prepared samples have been characterized by X-ray diffraction (XRD), Rietveld refinement of XRD pattern, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and charge/discharge tests. The results show that the spherical FeF2.5·0.5H2O–MWCNTs nanocomposites present the cubic crystal structure with the cell volume of 1.13293 nm3. Furthermore, it can deliver a high initial discharge capacity of 324.7 mA h g−1 and stable cycle performance of 175.2 mA h g−1 after 50 cycles at 40 mA g−1 between 1.5 V and 4.5 V. Especially, even at a high current density of 500 mA g−1, the as-prepared material can still display a high discharge capacity of 136.0 mA h g−1.

A first-principle study of the effect of OH− doping on the elastic constants and electronic structure of HTB-FeF3

FeF3 with a hexagonal-tungsten-bronze structure (HTB-FeF3) belongs to one type of promising cathode material for Li-ion batteries. However, poor conductivity has hindered its use for further practice. Moreover, its fundamental information, such as its geometrical structure, elastic properties and electronic structure, can rarely be found. In this study, first-principles calculations were implemented to perform a systematic investigation about the effect of OH− doping on orthorhombic HTB-FeF3. First, the single crystal elastic constants of HTB-FeF3−x(OH)x (x = 0, 0.083, 0.167, 0.333, 0.667, 0.833) were successfully obtained from the energy-strain curve calculations. The bulk and Youngs modulus, as well as the Poissons ratio for ideal polycrystalline HTB-FeF3−x(OH)x were calculated. The directionally dependent Youngs modulus of HTB-FeF2.167(OH)0.833 and HTB-FeF3 was also further calculated to analyze their elastic anisotropy. These results show that the partial substitution of OH− for F− ions can improve the elastic property of HTB-FeF3. Moreover, HTB-FeF3−x(OH)x can be classified as a ductile material. Furthermore, the ductility of polycrystalline HTB-FeF3−x(OH)x can be enhanced with increasing OH− doping concentrations. In order to clarify the mechanism of OH− doping on the orthorhombic HTB-FeF3, the crystal structure and chemical bonding properties of the orthorhombic FeF2.167(OH)0.833 and HTB-FeF3 crystals were further investigated. The calculated results indicate that strong O–H⋯F hydrogen bonds play a crucial role on the strengthening effect in HTB-FeF2.167(OH)0.833. Moreover, OH− doping can improve the electronic conductivity of HTB-FeF3.